1. Field of the Invention
The present invention relates to an optical amplifier which amplifies signal light in an optical waveguide to which pumping light is supplied, and an optical communication system including the optical amplifier.
2. Related Background Art
An optical communication system transmits large-capacity information with a high speed in such a manner that signal light having a plurality of channels of different wavelengths from each other (WDM: Wavelength Division Multiplexing signal light) propagates through an optical fiber transmission line. The C band (1530 nm to 1565 nm) has been already used as a signal wavelength range in the optical communication system, and the use of the L band (1565 nm to 1625 nm) also is considered. Further, in order to develop further large-capacity of information, and the use of the S band (1460 nm to 1530 nm) is considered as a signal wavelength range.
In the optical communication system, an optical amplifier to amplify signal light is applied. As an optical amplifier which enables to amplify the signal light of the C or L band, utilized is an EDFA (Erbium-Doped Fiber Amplifier) in which an optical amplification fiber (EDF: Erbium-Doped fiber) where Er (erbium) element is doped to its optical waveguide region is applied as an optical amplification medium. The EDFA can amplify signal light of the C or L band which propagates through the Er-doped optical fiber by supplying pumping light (0.98 xcexcm wavelength band or 1.48 xcexcm wavelength band) to the Er-doped optical fiber.
On the other hand, as an optical amplifier which enables to amplify the signal light of the S band, considered is TDFA (Thulium-Doped Fiber Amplifier) in which an optical amplification fiber (TDF: Thulium-Doped Fiber) where Tm (Thulium) element is doped to its optical waveguide region is applied as an optical amplification medium. The TDFA can amplify the signal light of the S band which propagates through the Tm-doped optical fiber by supplying pumping light (1.05 xcexcm wavelength band, 1.4 xcexcm wavelength band, or 1.55 to 1.67 xcexcm wavelength band) to the Tm-doped optical fiber.
In such an optical amplifier, it is important that its gain spectrum is flattened in the wavelength range of signal light to be amplified irrespective of the temperature change. For example, it is required that the gain of an optical amplifier employed in a land-based optical communication system is flat at least within the range of atmosphere temperature: 0xc2x0 C. to 65xc2x0 C. irrespective of the temperature. But, the gain spectrum of the optical amplification medium in the optical amplifier is typically not flat in the signal wavelength range, and the gain spectrum may shift toward a short or long wavelength side depending on the temperature. Therefore, it is required that the optical amplifier includes not only a gain equalizing filter for equalizing the amplification gain of signal light in an optical amplification medium but also a temperature compensator for reducing temperature dependency of the gain.
The inventors have studied conventional optical communication systems in detail and, and as a result, have found problems as follows.
After studying the aforementioned prior art, The inventor found the following problems. For example, according to a temperature compensator of EDFA disclosed in Japanese Patent Application Laid-Open No. 4-11794, it is intended to reduce temperature dependence of the gain by controlling the temperature of an optical amplification fiber itself by temperature adjusting means such as Peltier element. But, there exits a problem in that the electric power to drive the temperature adjusting means is large.
As another temperature compensator, by controlling the transmission characteristics of a variable optical attenuator, a technique which intends to reduce temperature dependency of the gain is also known. But, there exists a problem in that the controlling is complicated.
Incidentally, any temperature compensator of the optical amplifier known so far has EDFA as an object; however, the one having TDFA as an object is unknown. The present invention is made to solve the foregoing problems. It is therefor an object of the present invention to provide an optical amplifier (TDFA) including a configuration which enables to reduce temperature dependency of the gain with simple control, and an optical communication system including the optical amplifier.
In order to achieve the above-mentioned object, an optical amplifier according to the present invention is adapted for amplification of signal light in a wavelength range of 1455 to 1485 nm, and has an optical waveguide where Tm element is added to its optical waveguide region, a pumping light supply system for supplying pumping light to the optical waveguide, and a gain equalizing filter which is optically connected to the optical waveguide. More specifically, in the optical amplifier according to the present invention, the gain equalizing filter is characterized by having a loss spectrum which shifts toward the short wavelength side as the temperature of the optical waveguide is higher. Incidentally, when said optical waveguide is an optical fiber, at least its core region is included in the optical waveguide region doped with Tm element.
In accordance with the optical amplifier, pumping light is supplied from the pumping light supply system to the optical waveguide where Tm element is added to its optical waveguide region. When signal light of a predetermined wavelength range enters the optical waveguide, the signal light will be amplified in the optical waveguide. The loss spectrum of the gain equalizing filter which is optically connected to the optical waveguide has almost the same form as the gain spectrum of the optical waveguide. The amplification gain of the signal light in the optical waveguide is equalized by the gain equalizing filter, and the gain spectrum of the entire optical amplifier becomes flat in a predetermined wavelength range. The gain spectrum of the optical waveguide doped with Tm shifts toward the short wavelength side as the temperature of the optical waveguide of the optical waveguide is higher. On the other hand, the gain equalizing filter has a loss spectrum which will shift toward the short wavelength side as the temperature is higher. Therefore, the temperature dependence of the gain in the entire optical amplifier will be reduced in a predetermined wavelength range. In such away, the optical amplifier can reduce its power consumption and further reduce temperature dependence of the gain with simple control.
In addition, in the optical waveguide according to the present invention, the pumping light supply system may supply light of 1.05 xcexcm wavelength band only as the pumping light. At the time, the temperature coefficient of wavelength-shifting in the loss spectrum of the gain equalizing filter is preferably xe2x88x920.02 nm/xc2x0 C. The is because Tm element added to the optical waveguide region of the optical waveguide is subjected to up-conversion pumping by the pumping light of 1.05 xcexcm wavelength and, while the temperature coefficient of wavelength-shifting in the gain spectrum of the optical waveguide is xe2x88x920.02 nm/xc2x0 C. Therefore, the temperature coefficient of wavelength-shifting in the loss spectrum of the gain equalizing filter is xe2x88x920.02 nm/xc2x0 C.; as a result, the temperature dependence of the gain in the entire optical amplifier is reduced.
In addition, in the optical amplifier according to the present invention, the pumping light supply system may supply light of 1.05 xcexcm wavelength band and 1.55 to 1.65 xcexcm wavelength band to the optical waveguide as the pumping light. In this case, the temperature coefficient of wavelength-shifting in the loss spectrum of the gain equalizing filter is preferably less than xe2x88x920.02 nm/xc2x0 C. The is because the Tm element added to the optical waveguide is pumped by the pumping light of 1.05 xcexcm wavelength band and 1.55 to 1.65 xcexcm wavelength band, while the gain spectrum of the optical waveguide will shift toward the long wavelength side by the pumping light of 1.55 to 1.65 xcexcm wavelength band. Here, the temperature coefficient of wavelength-shifting in the gain spectrum of the optical waveguide becomes less than xe2x88x920.02 nm/xc2x0 C. Therefore, the temperature coefficient of wavelength-shifting in the loss spectrum of the gain equalizing filter is xe2x88x920.02 nm/xc2x0 C.; as a result, temperature dependence of the gain in the entire optical amplifier is reduced.
The optical amplifier according to the present invention may further include a control circuit for controlling wavelength-shifting in the loss spectrum of the gain equalizing filter. In this case, the loss spectrum of the gain equalizing filter is easily controlled when a stress or thermal energy, for instance, is applied thereto from the external.
Further, in the optical amplifier according to the present invention, the loss spectrum of the gain equalizing filter may be automatically wavelength-shifted without applying a control to the gain equalizing filter from the external. In this case, the loss spectrum is automatically wavelength-shifted without applying an electrical control, for instance, to the gain equalizing filter from the external, resulting in reduced power consumption.
The optical amplifier according to the present invention further includes a variable optical attenuator which is optically connected to the optical waveguide. The attenuation of the variable optical attenuator to signal light is variable in accordance with a temperature change. In this case, not only the gain spectrum of the entire optical amplifier in a predetermined wavelength range becomes flat irrespective of the temperature, but also the gain level thereof becomes constant.
An optical communication system according to the present invention includes an optical amplifier (optical amplifier according to the present invention) having the aforementioned configuration, and transmits the signal light while amplifying the multiplexed signal light (WDM signal light) propagating through a predetermined repeated section. According to the optical communication system, the signal light of a predetermined wavelength range (1455 to 1485 nm) will be amplified by means of the above optical amplifier. Therefore, a high transmittance-quality to signal light of a predetermined wavelength range will be ensured.
The present invention will be more fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.